Most radio sources are two-sided, like Cygnus A. A minority, however, are one-sided, and the first-known and brightest example is 3C273 (see Fig. 1), a high-luminosity QSO, showing ‘super-luminal’ proper motions in the core. The explanation of such one-sided sources may follow one of two lines (and it seems that both schools of thought are represented at the present meeting): on the one hand, the ejection of material from the central object may truly be one-sided, while on the other hand the ejection may be two-sided but at a relativistic speed, so that the receding half is hidden by Doppler beaming.